Self-excited linear dc amplifier having a bridge input portion



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United States Patent 3,408,584 l SELF-EXCITED LINEAR DC AMPLIFIER HAVING A BRIDGE INPUT PORTIONA `Edward J. Miller, Jr., Tempe, and George A. Prell, ,Phoenix, Ariz., assignors to Motorola, Inc., Chicago, Ill., va corporation of Illinois Filed Oct. 14, 1963, Ser. No. 315,997 6 Claims. (Cl. 330-9) ABSTRACT 0F THE DISCLOSURE An AC symmetrical bridge network having a varactor ode in each In the embodiment to -be described, the improved incorporated in the transmitter of an Parent 3,283,239' filed Swartwout et al. The

in the name of Charles J. aforesaid bridge network inmonitored by the control The basic input to the amplifier to be described, therefore, is the motion of the from the' bridge to develop the bridge.

3,408,584 Patented Oct. 29, 1968 tions, the bridge is sl'ghtly unbalanced so that a low amplitude alternating current error current excitation to the bridge. l n improved industrial process control system is d e-l scribed, f r example, in copending application Ser. No. 315,940,

it is con- .for use with a two-wire line between the process area and the control room. This line supply in the control room.

An lobject of the invention is is an accurate direct function of Another object of the invention is to improved ampliler system which finds in industrial process control systems, a two-wire transmission circuit to be trol system.

vA feature of the invention is the permits the use of a measurement bridge netprovde such an particular utility and which permits used in such a conrequired for the accurate balancing of bridge network.

Another feature of the invention is the provision of such an amplifier which is particularly constructed to operate n conjunction with a of line resistance irirthe installation of the amplifier inim suel transmitter.

Still another feature of the invention is the provision of such an improved amplifier which, as noted, is con- 'ceived and constructed to operate directly from acommon power supply at the control room area.

The oscillator-amplifier system of rthe invention supplies alternating current to the aforesaid measurement bridge network, as mentioned above, and it also acts as a bridge-balance error detector. The `measurement bridge network, as will be described, is balanced by a direct current feedback voltage which is developed across a precision feedback resistancein the `main direct current signal current path of the system. Since all the transmitter 'current fiows through the feedback resistor, a 'precise measurement of the transmitter output is provided; This direct current voltageis applied to `the junction of a pair of'voltage-variable capacitors which, as will be described, are included in the measurement bridge network. The system to be described includes a highgain alternating'current oscillator-amplifier and a highgain direct current amplifier which are necessary accurately to balance the measurement bridge.

Zero is adjusted in the system to be described by setting the initial direct current bias at the variable capactor junction of the bridge to a selected value. In the system to be described, the feedback voltage is negative with respect to the amplifier common reference potential, and the zero adjustment potential positive. The feedback voltage is summed with the zero-adjusted positive bias to give a positive signal voltage for the bridge.

A span control is provided in the amplifier system to be described by the provision of manually adjustable potentiometers across the aforesaid feedback resistance. This means that all the necessary adjustments of the system may be carried out by electrical rather than mechanical controls. This permits suitable external control of the system with the housing thereof intact.

Yet another feature of the invention therefore, is the provision of such an improved amplifier for use in the transmitter of an industrial process control system, and which is constructed so that all necessary controls are effectuated electrically, so that the system can be controlled with the housing intact so as to be safe, even when installed in a hazardous atmosphere.

The principal objects and advantagesof the invention therefore are to provide an improved measurement bridge amplifier system for particular use in the two-wire type of industrial process control system; and a system which is precise and accurate in its operation, in which all the controls are achieved electrically so that the system is relatively simple and inexpensive, and so that units incorporating the system may be relatively light and capable of intrinsically safe operation and adjustment externally of the units in hazardous atmospheres susceptible to explosion.

Other features, objects and advantages of the invention will become apparent from a consideration of the following description, when the description is taken in conjunction with the accompanying drawings, in which:

FIGURE l is a schematic representation illustrating in block and circuit form `one embodiment of the irnproved amplifier of the invention; and l FIGURE 2 is a circuit representation of one embodiment of the amplifier. y As mentioned above, the amplifier system to be described incorporates a solid state measurement bridge network and associated high gain alternating current and direct current amplifier stages. This bridge network is designated as in FIGURE 1, and it is described and claimed in copending application Ser. No. 316,012, filed Oct. 14, 1963, and now Patent 3,283,239.

The bridge network includes a variable transformer 12, and it also includes a pair of voltage sensitive capacitors 14 and 16. The capacitors 14 and 16 are actually a particular type of diode, known to the art as varactor.

Theserdiodes respond to vvoltage variations to exhibit corresponding changes in capacity.

The variable transformer 12 may be incorporated in a sensing device, such as in a pressure differential transducer, which constitutes a part of the transmitter. Such a transducer is described and claimed,for example, in c'opending application Ser'."No.' 3l5,82r1, filed Oct. ,14, 1963, and inow Patent 3,277,719. 'The transducer .described in the copending application incorporates a variable transformer .suchas the transformer 12. Relative motion between the stationary cores and the movable ar-mature 13 in response to external force applied'to the transducer results in a change in the inductances of the two halves 15 and 1S of the transformer circuit. "This, in turn, produces corresponding changes in the' 'amplitude ratio of the secondary signals E2 and E3, which=produces acorresponding unbalancing of the'bridge 10.'

The center tap of the secondary winding of the transformer 12 is shown as being connected to a point of reference potential, such` as ground, and the signals E2 and E3 appear on either side of the center tap. The secondary winding is connected to a pair of direct current isolating capacitors 18 and 20. The capacitors, in turn, are connected to a corresponding pair of alternating current isolating resistors 22 and 24. The junction of the capacitor 18 and resistor 22 is connected to the varactor 14, and the junction of the capacitor and resistor 24 is connected to the varactor 16.

The varactors 14 and 16 are connected together, and through an alternating current isolation resistor 25 to a sum-ming network 26. The junction of the varactors also is coupled through a capacitor 27 to the input of an alternating current oscillator-amplifier 28. The alternating current output from the amplifier 28 is fed back to the primary of the variable transformer 12. The output from the oscillator-amplifier 28 is rectified in a rectifier 30, and the resulting direct current signal is amplified in a direct current amplifier 32.

The resistors 22 and 24 are connected to a Zener diode 34 which serves as a voltage regulator. A pair of potentiometers 36 and 38r are connected across the Zener diode. The potentiometer 36 provides a fine zero adjustment, and the potentiometer 38 provides a coarse zero adjustment.

The arm of the Apotentiometer 38 is connected to the DC summing point 26 through a DC summing resistor 40. The Zener diode 34 is connected in the circuit of the amplifier 28, and'the anode of the Zener diodeV regulator is connected to a common lead 42. The lead 42 also serves as a common lead for the direct current amplifier 32.

The output circuit of the direct current amplifier 32 is connected to a lead 44, and through a precision feedback resistor 46 and a resistor 48 to a lead 50. The leads 44 and 50 constitute a two-wire transmission line, which eX- tends from the process area to the control room area. The source of direct current exciting potential for the system may be located in the control room area. The wire 44 is connected to the positive terminal ofthe source, and the wire 50 is connected to an output terminal 53 and through a precision resistor S2 to the grounded negative terminal of the source. An output controlvoltage is developed across the resistor 52. The potential source in a constructed embodiment of the invention is a 24-volt source, and the output control voltage varies in a range of from 1-5 volts direct current. The amplifier draws 4-20 milliamperes from the source.

A potentiometer 54 and a potentiometer 56 are connected across the precision resistor 46. The potentiometer 54 constitutes a fine span adjustment for the system, and the potentiometer 56 constitutes a coarse span adjustment. The arm o f the potentiometer 56 is connected through a DC summing resistor 58 to the summing point 26.

The variable transmormer in the transducer head forms half of the measurement `bridge network 10. AThe other half of the bridge consists of the two voltage-variable capacitors (varactors) 14 and 16. A voltage proportional to the transmitter output appears across the precision feedback resistor46, and the potentiometers 54 and 56. The feedback voltage (V1) is negativewith respect to This kvoltage is fed back to the voltage-variable capacitors the common amplifier lead 42. This voltage is summed 14 and 16 by way of the summing network 26, and serves with a fixed positive voltage derived from the zero adto rebalance the bridge The solid state rebalancing cirjustment potentiometers 36 and 38 to render the voltage cuit is highly stable and it is free from vibration effects 5 applied to the varactors 14 and 16 positive ,with respect normally associated with mechanical rebalancing systo the lead 42. l tems. v The alternating current amplifier 28 and the direct cur'- As mentioned above, the transformer 12 1's excited by rent amplifier 32 are high gain amplifiers so that the bridge the alternating current derived fromv the oscillator-amplimay be accurately balanced.

fier 2 8. lfthe movable core is symmetrically disposed 10 The amplifier system of FIGURE 1, therefore, produces with respect to the two portions of the secondary, `the voltage E2 will equal the voltage E3. If the armature is which currents correspond precisely and as a direct funcmoved 'away from the neutral position (as the transducer tion of the vmechanical input of the system. The resistance senses changes in theI variable) there willbe a change of these leads has no material effect on the operation `of in the ratio ofthe voltages h2y andvE3. For limited core 15 the transmitter, because in each instance the direct current travel, the ratio between the voltages E2 and E3v will be a amplifier 32 draws sufficient current through the loop to linear function of core travel This ratio will be unaffected rebalance the bridge Therefore there is no necessity to by the magnitude of the alternating current feedback voltmake allowances in the transmitter circuit for the resistage 1. ance of the two-wire line between the transmitter and the The ,voltage E4 equals the sum of the voltages E2 and 20 control room. Also, the particular circuit of FIGURE l`is V3.l For limited core travel, the voltage E4 will be essensuch that the transmitter in the process area may operate tially constant in amplitude assuming a constant input directly from a common power supply in the control amplitude El, The voltage E4`is the alternating current room area. V eiicitation on the other two legs of the bridge comprising The actual circuit of one embodimen-t ofthe invention is the voltage-variable capacitors (varactors) 14and 16. 25 shown in FIGURE 2.

The introduction of. la direct current input voltage The bridge network 10 is shown in more detail in FIG- (Vl) through the resistor 58 (summed with the voltage URE 2, and it includes a pair of additional diodes such derived through the resistor from the zero adjustment as the diodes 60 and 61 These diodes may be, for example, potentiometer 38) to the junction of the varactors 14 and silicon diodes, varactors and the like The diodes 60 and 16 results in a change in relative capacity of the varactors 30 61 are included in the bridge network forrtemper-ature as a function of the amplitude of this voltage That is an `compensation purposes, as described in more detail in increase in the voltage (V1) causes one of the varactors copending application Ser No 316,012, filed Oct 14 14 and V16 to increase its value while the other decreases 1963, and now Patent 3,283,239.

l The diode 60 is connected to the junction of the 4resistor The voltage (V1) can be adjusted so that the alternating 35 22 and a capacitor 62 through a resistor 63. The capacitor Current bridge output applied to the oscillator-amplifier 28 18 may have a capacity of .01 microfarad, the resistor 22 can be brought back to a null for any position of the may have a resistance of 100 kiloohms, the resistor 63 movable core. may have a resistance of 31.6 kiloohms, and the capacitor The operation of the bridge network is described in 62 may have a capacity of .01 microfarad. more detail in copending application Ser. No. 316,012, 40 The junction of the resistor 22 and capacitor 62 is conf'iled Oct., 14, 1963, and now Patent No. 3,283,239. nected through a resistor 65 to the junction of resistor 24 As mentioned above, the movable armature of the and capacitor 70 The varactors 14 and 16 are coupled to transformer 12 is moved as a function of variations in the alternatiny current null line through a capacitor 66 the variable being sensed by the tiansducer in which the and a resistor 67. The capacitor 66 may have a capacity transformer 12 is incorporated In the particular embodiof .001 microfarad, the resistor 67 may have a resistance ment described in the copending application Ser No 315 of 56.2 kiloohms, 'and the resistor 65 may have a resistance 821, filed Oct 14 1963 yand now Patent No 3 277,719 of 215 kiloohms. the armature is moved precisely as 'a function of dif- The diode 61 is connected through a resistor 69 to the' ferential pressure. junction of the resistor 24 and a capacitor'70. The capaci- The resulting unbalance of the bridge, causes a variator 20 may have a capacity of .O1 microfarad, the resistor tion in the amplitude of the alternating current output sig- 24 may have a resistance of 100 kiloohms, the resistor 69 nal developed by the amplifier 28 This, in turn, causes may have a resistance of 46 4 kiloohms and the capacitor a variation in the current drawn by the direct current 70 may have a capacity of 01 microfarad The capacitors amplifier 32 through the feedback resistor 46 The re- 62 and 70 are connected to the common line 42 of the to be automatically varied until the balance position of 69 are included in the circuit so as to reduce the compenthe bridge is again established. A sating bias changes imparted by the diodes and6l The rebalancing circuitprovides a precise electrical outto exactly the ranges required to compensate ,precisely for put in the `form of a direct current signal drawn through the characteristic changes of the varactors 14 and 16 with, the. conductor 50, and through the precision resistor 52 60 temperature. The resistors 63 .and 69`also serve to comat the control room area As mentioned, this rebalancing pensate for changes in the values of resistors 72 and 71 is accomplished without the use of any moving parts with temperature and they have mutually different resiste The zero position of the bridge can be adjusted by adance values for -that purpose. justments of the coarse potentiometer 38 and fine potentie The fine zero adjustment potentiometer 36 is connected ometer 36. In addition, the span can be adjusted by ad 65 to the lead 74. The junction of the resistors 71 and 7,2 is justment of the fine potentiometer 54 and coarse potentconnected to the varactor 60. The resistors 71 and 72 ometer 56. serve as -a voltage divider, and they permit adequate volt- It is apparent that substantially all the transmitter outage to be applied from a common source to the zero adput direct current flows through the precision feedback justment potentiometers 36 and 38 without excessive voltresistor 46, sov that th'e voltagedeveloped across that re- 70 age being applied to the bridge 10. The resistor 71 may sistor is a precise measure of the transmitter output Also, ave a resistance of 33 2 kiloohms and the resistor 72 the use of the span adjustment potentiometer S4 and 56, may have a resistance of 13 3 kiloohms The coarse zero permit any desired portion of the voltage across the readjustment potentiometer 38 is connected to the common sistor 46 to be fed back to the bridge network, so that lead 42 through a resistor 75. apconvenient span adjustment is provided. 7e', Y The resistors 40 and 58 are connected through a resis- 7 the varactors 14 and 16. The fine zero adjustment potentiometer 36 may have a resisttor 76 to the junction of ance of 500 ohms, and the coarse zero adjustment potentiometer 38 may have a resistance of 25 kiloohms. The resistor 75 may have a resistance of 33.2 kiloohms, the coarse span adjustment potentiometer S6 may have a resistance of kiloohms, and the fine span adjustment potentiometer 54 may have a resistance of 500 ohms. The resistor 58 may have a resist-ance of 249 kiloohms, the resistor may have a resistance of 178 kiloohms, and the resistor 76 may have a resistance of 100 kiloohms.

A stabilizing capacitor 77 is connected to the movable arm of the coarse span adjustment potentiometer 56. This capacitor may have a capacity of .022 microfarad. A capacitor 78 and a series resistor 79 are connected between the lead 42 and the junction of the resistors 58 and 76. The resistor 79 and the capacitor 78 form a resistancecapacitance stabilizing network for the bridge loop circuit. The capacitor 78 may have a capacity of .022 microfarad, and the resistor 79 may have a resist-ance of 10 kiloohms. The diode 61 is connected to the common amplifier lead 42. The transformer primary is AC grounded through capacitor 80. If the other side of this transformer is grounded and the secondary leads to capacitors 18 and 20 are reversed, the DC output may be made to decrease with pressure. This reverse action is sometimes desirable in a process control transmitter and is an important feature of this unit.

The alternating current null line is connected to an input terminal 101 of the alternating current amplifier 28. The amplifier 28 is a usual alternating current amplifier with internal degenerative feedback being provided for stabilizing purposes. This terminal is connected to the junction `of a pair of voltage-divider biasing resistors 102 and 104, and to the base electrode of a transistor 105. The resistor 102 is connected to a lead 106, and the resistor 104 is connected to the common amplifier lead 42. The resistors 102 and 104 may each have a resistance, for example, of 220 kiloohms.

The collector of the transistor 105 is connected to a resistor 108, and to the base electrode of a transistor 125. The resistor 108 may have a Iresistance, for example, of 27 kiloohms, and it is connected to the lead 106. The emitter of the transistor 105 is connected to ra feedback resistor 110 and to a resistor 112. The resistor 110 may have a resist-ance, for example, of 27 kiloohms, and it is connected to a capacitor 114. The resistor 110 and capacitor 114 form an alternating current degenerative feedback circuit for alternating current gain stability.

The capacitor 114 may have a capacity, for example, of .47 microfarad, and it is connected to the collector of the transistor 125. The resistor 112 may have a resistance of 270 ohms, for example, and it is connected to a resistor 116. The resistor 116 may have a resistance, for example, of 39 kiloohms, and it is connected to the common amplifier lead 42. The resistor 116 is shunted by a capacitor 118 `having a capacity, for example, of .47 microfarad.

The collector of the transistor 125 is connected through ya resistor 120 to the lead 106. The resistor 120 may have a resistance, for example, of 27 kiloohms. The collector of the transistor is also connected to a resistor 122. The resistor 122 is connected to an alternating current coupling capacitor 124 which, in turn, is connected to the base of -a transistor 137. The resistor 122 may have a resistance of 100 ohms, and the capacitor 124 may have a capacity of .47 microfarad.

The emitter of the transistor 125 is connected to a resistor 126. The resistor 126 may have a resistance of 220 kiloohms. This resistor is connected to the lead 42. A capacitor 128 is shunted across the resistor 126. The capacitor 128 has a capacity, for example, of .47 mircofarad. A limiting resistor 129 is connected to the lead 106 for intrinsic safety purposes. This resistor may have a resistance of 100 ohms. A capacitor 130 is connected to the resistor 129 and to the lead 42. The capacitor 130 may have a capacity of 4.7 microfarads.

The base of the transistor 137 is connected to a direct current feedback resistor 132 and to a resistor 134. The resistor 134 has a resistance of 52.2 kiloohms, and it is connected to the lead 42. The feedback resistor 132 has a resistance of 17.8 kiloohms, and it is connected to a capacitor 136. The capacitor 136 is connected to the lead 42, and it has a capacity, for example, of 15 microfarads.

The collector of the transistor 137 is connected to a resistor 140. The resistor 140 has a resistance of 12.1 kiloohms. The resistor is connected to a lead 142. A decoupling resistor 144 is connected to the resistor 129 and to the lead 142. The resistor 144 may have a resistance, for example, of 15 kiloohms.

The emitter of the transistor 137 is connected to a Capacitor 146 which, in turn, is connected to the lead 42. The capacitor 146 has a capacity, for example, of 15 microfarads. The emitter of the transistor 137 is also connected to a resistor 148 which is connected to the lead 74 to supply the regulated direct current bias voltage to the bridge network 10 from the Zener diode 34. The resistor 148 has a resistance, for example, of 100 ohms.

The collector of the transistor 137 is connected to the base of a transistor 139. The transistors 105, 125, 137 and 139 are all included in the high gain alternating current oscillator-amplifier 28. The emitter of the transistor 139 is connected to a resistor 150 and to a resistor 152. The resistor 150 has a resistance of 9.65 ohms, for example, and is connected to a second resistor 154. The resistor 154 may have a resistance of 2.5 kiloohms, for example. This resistor is connected to the cathode of the Zener diode 34, the cathode of the Zener diode being further connected to the capacitor 146 and to the resistor 148. The anode of the Zener diode 34 is connected to the lead 42.

The resistor 132 connects the junction of the resistors 150 and 154 back to the base of the transistor 137 to provide a degenerative feedback circuit for regulating the current through the Zener diode 34.

The resistor 152 has a resistance, for example, of 499 kiloohms, and is connected to a capacitor 158. The capacitor 158 has a capacity, for example, of .047 microfarad. This capacitor is connected back to the base of the transistor 105 to constitute an alternating current degenerative feedback circuit for stabilizing the current gain of the oscillator-amplifier 28. This circuit picks current off the emitter of the transistor 139 of the final stage of the amplifier and feeds it back to the base of the transistor 105 of the first stage. The degenerative feedback circuit also serves to stabilize the input impedance of the amplifier in that it makes the input impedance of the base of the transistor 105 virtually to the ground level.

The collector of the transistor 139 is connected to a tap on an inductive winding 160. The winding 160 is shunted by a pair of capacitors 162 and 164, and by a resistor 166. The resistor 166 may have a resistance of 17.8 kiloohms, and each of the capacitors 162, 164 may have a capacity of .0091 microfarad. The capacitors 162, 164 and the inductive winding 160 forrrr a parallel resonant tuned tank circuit for the oscillator-amplifier 28. This tank circuit is connected to the lead 142. The tank circuit is tuned to a selected frequency, such as 30 kilocycles. Oscillations at that frequency are set up in the system.

An inductive winding 170 is inductively coupled to the winding 160, and the winding 170 is connected back to the primary of the variable transformer 12. This winding supplies alternating current to the bridge network 10, and also serves to establish an oscillatory condition in the system.

The null condition in the system is set by the setting of the fine and coarse zero adjustment potentiometers 36 and 38. This provides for a low amplitude alternating current signal at the input terminal 101. This signal is amplified in the alternating current amplifier 28 and fed back to the bridge by the above-described positive feedback loop. Then, any variations in the movable tween the input and output of the system. The transistor 210 constitutes the fier 32.

The emitter of the transistor 210 is connected to the center tap of the winding 200, and the emitter is also connected to a load resistor 212 and to the base of an NPN transistor 214. This latter transistor constitutes the second stage of the direct current amplifier 32. The load resistor 212 may have a resistance, for example, of 4.7 kiloohm's, and it is connected to the lead 42. A Zener diode 216 is connected between the lead 142 and the lead 42, for voltage regulating purposes. This diode may be shunted by a filter capacitor 218 which, for example, may have a capacitance of 4.7 microfarads.

The emitter of the transistor 214 is connected to a load resistor 220 which,

232. The capacitors 230 and 232 lead 42.

are connected to the No. 315,940, filed Oct. 14, 1963, and signee of this application. As discussed in the copending application,

in the presence of short circuits in the system.

234 has its cathode connected tothe junction of the resistor 226 and coil 228 and has its anode connected to the lead 42. The resistor 226 may have a resistance of 100 ohms, the capacitor 230 may have a capacity of .22 microfarad, and the capacitor 232 may have a capacity of 27 microfarads.

The coil 224 is included in the voltage boost circuit which is described in more detail in copending application connected to a resistor fying diode of this application, and now abandoned, the signal from tion, as described in the copending case.

The capacitor 232 is an energy storage capacitor which The lead 42 is connected to the negative side of the power supply through the resistors 46, 48 and 52. The outresistance of 52.6 ohms, and the resistor 52 may have a resistance of 250 ohms.

A capacitor 249 is shunted across the resistors 46 and for example, of

122 microfarads. An intrinsic safety limiting resistor 250 cated at the control room. The resistor 48 may have a of 60 ohms, and the choke coil with the sche- FIGURE 1, normally ows through the two-wire circuit 44 and 50, when the power supply and the preclsron resistor 52 are located at the control room.

Any variation of the transducer varies the inductance of the upper and lower halves 14 and 15 of the centertapped secondary of the transformer 12, so as to unbalance the bridge network 10. The

rectified and the resulting direct current signal is amplied in the direct current amplifier 32. This causes the direct current amplifier 32 either to increase or decrease the direct current drawn through the resistors 52, 43 and 46. means including rectifying meansand a direct current This increase or decrease causes the voltage across the 'v `amplifier coupled to said alternating current -amplfeedback resistor 46 to vary, so that the direct current fier for receiving Said amplified modulated aliernat. voltage applied tothe varactors 14 and 16 varies. The ing current Signal, eapeeitanee of the Yernetors 14 and 16 Changes aeeord- 5 an output circuit included in said direct current am' ingiy in a direetion to balance the bridge 10, and to return i plifier including a feedback resistor for developing the alternating Current Signal applied to thearnpliner 23 a direct current feedback voltage thereacross having to the null point. The net voltage fed back to the bridge an amplitude related to the amplitude of said modis controlled electrically for spari control purposes; and nlated alternating current signal, i the base voltage level is also controllable electrically for in circuit means including rnnnnnilyenjnstable Span con. Zero adlustrnent purpSeS- i trol potentiometer means connected in shunt with Therefore, the direct current flowing in the resistors 46, Vsaid feedback resister and having n tap 'electrically 48 and 52 represents the Current required to Produce the intermediate ends of said span control potentiometer required voltage to reset the bridge. This direct current is v rnesns and said circuit means being connected across a precise direct function of the movements of the transl i aid fourth afin with said tap being connected inducer in which the transformer 12. id incorpdratedtermediate said third and fourth arm for applying a The circuitry of-the inductance coil 224, diode 240, and direct current coltnge to said bridge ncross s'nid associated filter components 239 and 238, provide a voltfourth arrn for altering the reactive electrical char, age boost for the excitation of the transistors in the ampli- .actcristic of the elcctrical semiconductor device in fiel- 2 8 The voltage developed by the voltage bOoSt eir- 20 said fourth arm, to return said bridge toward its AC cud 1S added to the Power supply Voltage as described v balanced condition for reducing 4the amplitude of and claimed in the copending application Ser. No. 316,- `Snid modulated alternnting cnrrcnt Signal and 033,file d Oct. 14, 1963, assigned to the assignee of this ,moans Sunniying a Substantially constant voltnge mno application, and now abandoned. The use of the voltage nitnde across Said second portion for biasing Said boost crcult as described m the Copending case permits a 25 devices to exhibit said reactive electrical characterrelatively 10W voltage power Supp 1y to be Sed for ef' istics, and AC coupling means at each remote end c1ency-and sdfety purposes' of said first and second portions coupling said por- The invention provides, therefore, an improved amplitions together and reference notential means Con fier which is particularly suited for use in conjunction with nected to said bri(ige intermediate said iirst and the solid state measurement bridge 10. Moreover, the irnproved amplifier of the present invention has particular utility in two-wire transmission systems, such as described above.

The amplifier system of the invention is conceived and constructed, so that variations in a transducer-controlled input, produces corresponding direct current variations in a two-wire line circuit. The system is constructed so that `the resulting direct current variations precisely and accurately reflect the variations in the input transducer.

While a particular embodiment of the invention has 4() been shown and described, modifications may be made, and it is intended to cover in the claims all modifications -which fall within the scope of the invention.

We claim:

1. An amplifier system including in combination:

a bridge network having AC balanced and unbalanced conditions and including a first portion having first and second arms series-connected together, each arm having electrical means with an electrical characteristic and responsive to a variable effect to alter such thu-.d and fourth arms respectively. each with a electrical characteristic in accordance with said effect Variable mductamfe and having a pnmari.' Wmdmg for estabiisning the bridge network in an AC nn means coupled to said transformer for selectively altering said inductance by differing amounts, respecbalanced condition, and including `a second portion having third and fourth series-connected arms with tiVeiYifor estnbllslnng the bridge in z munbalanced cach of said third and fourth arrns having a scrni condition for modulating said alternating current exconductor device exhibiting an electrical characterciting Signal With the bridge unbalanceinereaSing istic and each semiconductor device being responsive die amplitude of the modulated alternatlng Current to an applied direct current voltage for .nltering its signal intermediatev said first and second series con- 'electrical characteristics which alternation affects the acted arms AC balance of Said bridge, an alternating current amplifier having an AC coupling AC means for supplying ari alternating current excitiiipiit Portion ineens Coupling Said bridge from ileing signal across said first portion of said bridge nettween Said rst and Second 'arms to Seid alternating work to cause said bridge to develop from said al- Current ernpiiiier Input Portion tor intr0du e1ng Said ternating current exciting signal a modulated amplimodulated 'alternating Current Signal to Said arnPll tude 'alternating current signal amplitude modified ner, by the AC balanced-unbalanced condition of said G5 a direct Current ainplilier having rectifier IneanS C0 u bridge network, the greater the unbalance the greater pled to Seid alternating Current aniplitier for reoeiV- the amplitude of said modulated alternating curing Said modulated alternating eurrentsignal as ernrent signal, plified by said alternating current amplifier, and supsecond legs.

2. The amplifier system defined in claim 1 in which said feedback resistor is included in a series loop circuit extending to a control area.

3. An amplifier system, including in combination:

va bridge network including a first pair of first and second series connected arms each having an electrical device, one electrically similar to the other, and each having a voltage variable capacitance characteristic and responsive to an applied direct current control signal for controlling the balanced-unbalanced condition of said bridge by altering said capacitance characteristic and further including a second pair of series connected third and fourth arms, input circuit means for introducing an alternating current exciting signal across said third and fourth arms of said bridge and including a transformer having a tapped secondary winding having equal portions of said winding included in each of said an alternating current amplifier having an input porplying a rectified 'signal indicative of. the bridge iintion coupled i0 said bridge network intermediate ro balance -and including an .Output Clrcult 1n Whlch said third and fourth arms for receiving and amplisaid rectified signal is supplied, i i fying said amplitude modulated alternating current further circuit means in said output circuit for derivsignal, and supplying such modulated alternating ing a direct current control signal from said recticurrent signal to said AC means for completing an fied signal and connected to said junction between oscillatory circuit, said first-and second arms of said bridge to intro- 13 duce said direct current control signal thereto for altering the capacitance characteristic so as to return the bridge substantially to the ybalanced condition,

AC coupling means coupling the ends of said first and third arms together and said second and fourth arms together, and means establishing a DC potential at the AC coupled ends of said first and second arms.

unbalanced condition.

translating system having an AC amplifier and supplying an AC signal to a rectifier which supplies rectified AC to a DC amplifier, the DC ment including in combination,

input means including a two electrically responsive impedance means series connected together respectively forming two other arms of the bridge network,

capacitance coupling means capacitively coupling the outer extremities of said other legs to said input means arm extremities,

an intermediate junction between the electrically re- 14 sponsive impedance means carrying said amplitude modulated AC signal, an electrical connection between said intermediate junction and an input of the AC amplifier for supplying said amplitude modulated AC signal thereto, means connected to said AC amplifier including voltage regnilator means for receiving said amplitude modulated AC signal and supplying a DC zero adjustment signal derived from said received AC signal,

sisting of work. 6. The system of References Cited UNITED STATES PATENTS 2,729,396 l/1956 Impey et al. 328`3 X 2,956,234 10/1960 Olsen S30-10 2,995,698 8/1961 Collins 324-98 X 3,040,157 6/1962 Hukee.

ROY LAKE, Prmw'y Examiner. NATHAN KAUFMAN, Assistant Examiner. 

